Cells in a MONOS (metal-oxide-nitride-oxide-silicon) type nonvolatile semiconductor memory device have recently been becoming finer. With finer cells, an leakage of charges from a charge storage layer is more likely to occur, thus rendering it difficult to ensure a voltage applied to a tunnel insulating film. Thus, a block insulating film made of a high-dielectric-constant material is used on the charge storage layer.
However, lanthanum aluminate, which has heretofore been used for the block insulating film, involves many defects because of having deliquescence. Therefore, the leakage of charges from the charge storage layer to an adjacent cell occurs, thus causing the problem of degradation in charge holding characteristics.